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OBJECTIVE: To study postpartum changes in pelvic floor morphology in a cohort of primiparous women.

METHODS: Transperineal ultrasound measurements taken at five examination points, both prepartum and postpartum, provided data for comparison. Three hundred nulliparous pregnant women were examined at 21 weeks of gestation and 274 (91%) at 37 weeks of gestation. At 6 weeks postpartum, 285 (95%) women were examined, 198 (66%) at 6 months, and 178 (59%) at 12 months using transperineal ultrasonography at rest, during contraction, and during Valsalva maneuver. The levator hiatus area, bladder neck mobility, and rest-to-Valsalva hiatal area difference were assessed.

RESULTS: Approximately 85% had vaginal and 15% had cesarean deliveries. Demographic characteristics of the patients lost to follow-up were similar to the patients not lost to follow-up. In the vaginal group, a significant decrease in all measurements was seen during the first 6 months postpartum, being most pronounced for the levator hiatus area during Valsalva maneuver (−3.5 cm2; P<.001). In the cesarean delivery group, no significant changes between examination points were found postpartum. Only the vaginal delivery group showed significant increases in all measurements when comparing the status at 12 months postpartum with 21 weeks of gestation, most pronounced for levator hiatus area during Valsalva maneuver (3 cm2; P<.001). However, comparing the two delivery groups at 12 months postpartum, the only significant difference found was levator hiatus area during contraction.

CONCLUSION: The levator ani muscle has the ability to recover after pregnancy and delivery, although not all women recover to pregnancy level. Most of the recovery occurs during the first 6 months postpartum. Significant pregnancy-induced changes are not shown to persist 1 year postpartum.

LEVEL OF EVIDENCE: II

The levator ani muscle recovers after the effects of pregnancy and delivery, although some delivery-induced changes are shown to persist 1 year postpartum.

Department of Obstetrics and Gynecology and HØKH, Research Centre, Akershus University Hospital, Lørenskog, and the University of Oslo, Faculty Division Akershus University Hospital, and the Department of Sports Medicine, Norwegian School of Sport Sciences, Oslo, Norway.

The pelvic floor muscles support the pelvic organs and play an important role in maintaining their position and function. The pelvic floor, or pelvic diaphragm, forms a sheet of muscle that spans the pelvic cavity with the levator ani muscle being an essential part.1 It allows for the passage of the urethra, vagina, and rectum through the levator hiatus. Many women experience pelvic organ dysfunction at some stage in their lives2 and vaginal delivery has been established as one of the most important risk factors associated with dysfunction later in life.3

However, it is not only events during parturition that can affect pelvic organ support. An increase in the size of the levator hiatus area has been shown to occur in the antepartum period.4–6 Women who go on to have caesarean deliveries may also experience loss of pelvic floor support, which strengthens the conclusion that changes that take place during pregnancy might cause this effect.7 We have previously reported pregnancy data from this cohort of women,4 but there is a lack of longitudinal imaging studies addressing possible changes in levator ani morphology after delivery compared with changes during pregnancy.

The aim was to study changes in pelvic floor morphology after both vaginal and caesarean deliveries using three- and four-dimensional ultrasonography and compare these changes with prepartum data in a cohort of primiparous women followed longitudinally and examined at five set points.

MATERIALS AND METHODS

This prospective cohort study was performed at Akershus University Hospital, Lørenskog, Norway, from January 2010 to October 2012. This study presents data from an observational study on pelvic floor changes during and after pregnancy and data analyzed in this study are a follow-up to previously presented data on this cohort of women during pregnancy.4 Nulliparous pregnant women were invited to participate when they attended their routine second-trimester ultrasound examination at 18–22 weeks of gestation. Inclusion criteria were singleton pregnancy, speaking a Scandinavian language, being older than 18 years of age with no prior pregnancy lasting more than 16 weeks, and no serious illness. Exclusion criteria were miscarriage, stillbirth, premature delivery before 32 weeks of gestation, new pregnancy of more than 6 weeks, or randomization to intervention with pelvic floor muscle training at 6 weeks postpartum.8 The Regional Ethics Committee (REK Sør-Øst D 2009/170) and the Norwegian Social Science Data Service (2799026) approved the study, and all participants gave their written informed consent to participate.

A power calculation was performed using results from a previous study9 in which a sample size of 47 was required to detect a 5% change in levator hiatus area at rest with a two-sided α of 0.05 and a power of 80%. We decided to include 300 women at 21 weeks of gestation to preserve the power, taking into account the possibility of women dropping out and exclusions resulting from women participating in a randomized controlled trial of postpartum pelvic floor muscle training.8 Demographic data and delivery data were collected from the women's electronic medical records and from a questionnaire exploring additional background data. All women underwent a three- and four-dimensional transperineal ultrasonography in the supine lithotomy position after voiding with a 4- to 8-MHz curved array ultrasonography transducer. Measurements were taken at rest, during pelvic floor muscle contraction, and during Valsalva maneuver by two trained examiners. The women were examined at 21 and 37 weeks of gestation and 6 weeks, 6 months, and 12 months postpartum.

Correct pelvic floor muscle contraction was taught by a physiotherapist and verified by observation of inward perineal movement and vaginal palpation. The Valsalva maneuver was performed for at least 6 seconds10; care was taken to avoid cocontraction of the most medial part of the levator ani muscle during the Valsalva maneuver.

Each maneuver was recorded three times and stored offline using anonymous code numbers. The examiners were blinded to the women's clinical data. Postpartum, the women's lower abdomen and pelvis was covered by a sheet and they were asked not to divulge any information regarding their deliveries.

All volumes were analyzed using 4D View 7.0 and 10.0. The levator hiatus area was measured as the area bordered by the most medial part of the levator ani muscle, the symphysis pubis, and the inferior pubic ramus using methodology as previously described.4 The bladder neck was measured in the midsagittal plane using a coordinate system as described by Peschers11 between rest and Valsalva maneuver. A higher value represents a more mobile bladder neck.

Major defects of levator ani muscle were assessed using tomographic ultrasonography imaging on the axial plane at maximal pelvic floor muscle contraction. The plane of minimal hiatal dimensions was used as a reference plane using the definition described by Dietz.12 Partial defects were defined as abnormal muscle insertions present in fewer than all three central planes.

Enlarged levator hiatus (also called ballooning) was defined as a levator hiatus area of more than 25 cm2 during the Valsalva maneuver.13 The ability of the levator ani muscle to distend was shown in the absolute difference between measurements of the levator hiatus area (cm2) at rest and during maximal Valsalva maneuver14 named rest-to-Valsalva hiatal area difference.

Volumes were analyzed by four trained investigators (with intraclass correlation coefficient greater than 0.80).4 Evaluations of major levator ani muscle defects were performed by two of the investigators. Interrater agreement between them was good to excellent (κ greater than 0.63).15

The assessors were blinded to the women's obstetric history and prior examinations. Images were analyzed in random order to avoid pairwise analysis, because it was not possible to blind the assessors to the number of weeks of gestation (eg, visibility of the fetal head on most images taken at 37 weeks of gestation).

SAS 9.3 and SPSS 20 were used for statistical analysis. Demographic data and ultrasound measurements were reported as means and standard deviations or frequencies and percentages. Normality of the data was assessed visually by histogram and QQ-plot. Independent samples t test and χ2 test (or Fisher's exact test) were used to compare continuous and categorical background variables between the study group and background population. The McNemar test was applied for a comparison of major levator ani muscle defects and enlarged hiatus over time. A linear mixed model, correctly adjusting for within-patient correlations as a result of repeated measurements, was used to assess the changes in ultrasound measurements from 21 weeks of gestation until 1 year postpartum. Random intercepts accounting for within-patient variability were included in the model. Dummy variables were created for each of the five examination points and included in the model as fixed effects together with the variable defining delivery mode. To assess the differences in changes within the two delivery mode groups, the interaction between dummies for time and delivery mode was included. Mixed models utilize all available information on participants in contrast to an analysis of variance. The strength of the linear mixed model is the ability to handle any degree of imbalance in data, often occurring as a result of participants dropping out or missing examinations. P<.05 was considered significant.

RESULTS

Three hundred pregnant nulliparous women were included at 21 weeks of gestation. Figure 1 shows a flow diagram illustrating the number of women examined at each consultation and the various reasons for nonparticipation. Of the 300 nulliparous pregnant women examined at 21 weeks of gestation, 274 (91%) were examined at 37 weeks of gestation. At 6 weeks postpartum, 285 (95%) women were examined, 198 (66%) at 6 months, and 178 (59%) at 12 months.

Background variables are presented in Table 1. Some of these data were originally presented in a cross-sectional study on continence and pelvic floor status at midpregnancy.16 Our study sample was comparable to the total population of nulliparous women scheduled to deliver at Akershus University Hospital during the inclusion period (n=2,621) with respect to age and marital and cohabitation status and delivery mode, but the women in the study sample had higher educational status (75.3% compared with 50.8%, P<.001).

We found no significant differences in background variables between the women who delivered vaginally and by cesarean delivery. Demographic characteristics of the patients lost to follow-up (n=122) were similar to the patients not lost to follow-up (n=178) with the exception of a slightly higher number of married and cohabitant women completing the study.

Figure 2 shows estimated mean levator hiatus area and bladder neck mobility for the different delivery groups from 21 weeks of gestation to 12 months postpartum. Table 2 shows the estimated mean changes in variables between examinations postpartum as well as between 12 months postpartum and pregnancy.

A significant change toward a smaller levator hiatus area at rest, during contraction, and during Valsalva maneuver and further a decrease in bladder neck mobility was found in the vaginal group from 6 weeks to 6 months postpartum. From 6 months to 12 months postpartum, no further significant change was found in these variables. For the women who had a cesarean delivery, no significant change was found among any of the examination points postpartum.

Comparison between delivery mode groups showed that women having delivered vaginally had significantly larger levator hiatus measures and increased bladder neck mobility both at 6 weeks and at 6 months postpartum when compared with women having a caesarean delivery (Table 3). At 12 months postpartum a significant difference between delivery mode groups was only found for levator hiatus during contraction.

At 6 weeks postpartum, major defects of the levator ani muscle were diagnosed in 26 of 179 (14.5%) women, all of whom had delivered vaginally; of these, 14 had delivered normally and 12 had an instrument-assisted delivery. No major defects were diagnosed in women who had a cesarean delivery. A significant reduction in diagnosed major levator ani muscle defects was seen from 6 weeks (26/179 [14.5%]) to 6 months (19/179 [10.6%]) postpartum (P=.016) with minor changes occurring between 6 and 12 months (16/179 [8.9%]). For the 10 defects no longer qualifying as major defects at 12 months, six were categorized as partial defects and four as normal. No de novo major levator ani muscle defects were diagnosed at 6 months or 12 months postpartum.

In the vaginal group there was a significant (P=.025) reduction in the number of women with enlarged levator hiatus between 6 weeks and 6 months postpartum (Table 4). No further reduction happened from 6 to 12 months postpartum. For the caesarean delivery group, the number of women with enlarged levator hiatus remained stable in the postpartum period.

The only significant difference between delivery groups in the number of women with enlarged levator hiatus was found at 6 weeks postpartum, in which more women having delivered vaginally had an enlarged levator hiatus (Table 4).

Measuring the rest-to-Valsalva hiatal area difference (Fig. 3), a significant reduction was found only in the vaginal delivery group from 6 weeks to 6 months postpartum with no further change from 6 to 12 months postpartum (Table 2). In group comparisons, the vaginal group showed a significantly greater rest-to-Valsalva hiatal area difference at 6 weeks postpartum than the cesarean delivery group (Fig. 3; Table 3). However, at 6 months and 12 months postpartum, the two delivery groups did not differ significantly.

When comparing levator hiatus measurements and bladder neck mobility at 12 months postpartum with measurements at 21 weeks of gestation, there were significant increases in all measurements in the vaginal delivery group (Fig. 2; Table 2). However, if comparing measurements at 12 months postpartum with those of 37 weeks of gestation, there was a significant decrease in the levator hiatus area at rest and during contraction, although not during Valsalva maneuver. In the vaginal delivery group, bladder neck mobility was significantly higher at 12 months postpartum than at 37 weeks of gestation. For the cesarean group no significant changes were seen when comparing levator hiatus measurements or bladder neck mobility at 12 months postpartum with 21 weeks of gestation. However, when comparing 12 months postpartum with 37 weeks of gestation, there was a significant decrease in all levator hiatus area measurements for the cesarean delivery group. No significant difference was seen for bladder neck mobility.

In the vaginal delivery group, the number of women with enlarged levator hiatus had increased significantly (P=.02) at 12 months postpartum when compared with 21 weeks of gestation (Table 4) but not when compared with 37 weeks of gestation. For the cesarean delivery group, the number of women whose levator hiatus became enlarged between 21 weeks of gestation and 12 months postpartum (Table 4) was greater than in the vaginal delivery group. However, this may not be statistically significant as a result of the small sample size.

Major defects of the levator ani muscle were diagnosed only postpartum in the vaginal delivery group, so compared with prepartum figures, there was a substantial increase 1 year postpartum.

The rest-to-Valsalva hiatal area difference was significantly increased for the vaginal delivery group when comparing 12 months postpartum with 21 and 37 weeks of gestation (Table 2; Fig. 3). For the cesarean delivery group, the rest-to-Valsalva hiatal area difference remained unchanged compared with pregnancy.

DISCUSSION

Our findings indicate that the levator ani muscle has an ability to recover from the effects of pregnancy and delivery; however, not all women recover to their pregnancy level. Most of the recovery happens during the first 6 months postpartum.

Strengths of the study include the longitudinal design, high interobserver reliability of data,17 and use of a linear mixed model, accommodating for imbalance in data and for correlations among repeated measures. Limitations include the limited number of cesarean deliveries and unavailability of prepregnant measurements. We have chosen .05 as the level of significance. However, to reduce the risk of false-positive findings resulting from multiple tests, findings with levels of significance above .01 should be interpreted with caution.

Most of the recovery of the levator ani muscle happens during the first 6 months after delivery. Magnetic resonance imaging studies report recovery during the first months postpartum,18,19 whereas others found no significant changes in levator hiatus dimensions from 4 months to 3 years postpartum.20 This is in accordance with our findings and could indicate that recovery takes place early postpartum, but the duration of recovery varies according to whether the changes were induced by pregnancy, delivery, or both, as seen in Figure 2.

We found increased bladder neck mobility after vaginal delivery with a tendency for recovery the first 6 months postpartum in accordance with several other studies.21–24 Diverging conclusions concerning bladder neck mobility after cesarean delivery have been published.21,24,25 However, most studies are in accordance with our results showing that bladder neck mobility is not significantly increased after cesarean delivery.

The largest change in the prevalence of visible major levator ani muscle defects was found during the first 6 months postpartum. Overdiagnosing early after delivery as a result of edema26 has been put forward as a possible reason for the decrease in numbers of visible defects seen later postpartum, but a natural healing process cannot be excluded.20,27

The rest-to-Valsalva hiatal area difference and the number of women with an enlarged levator hiatus both decreased most during the first 6 months after vaginal delivery. Interestingly, the number of women having an enlarged levator hiatus increased during pregnancy4 and at 1 year postpartum, no difference between delivery groups was found. An increased ability to stretch the muscle fibers and the connective tissue during pregnancy might help protect the muscle during parturition,28 although postpartum the same features have been put forward as an indicator of less firm support for the pelvic organs.14 Furthermore, it is not known whether an enlarged levator hiatus 1 year postpartum is associated with pelvic organ prolapse as it is later in life.29

Even if recovery takes place, the levator ani muscle does not seem to have recovered completely 12 months after pregnancy and vaginal delivery. Similar findings have been reported in another longitudinal study.14 Injury to the muscle, but also injury to the connective tissue and nerves, might contribute.

Contrary to others,14,21,24 we found no evidence that changes in levator ani muscle morphology seen during pregnancy, reflected as changes after cesarean delivery, persisted 1 year postpartum. In the cesarean delivery group, we found a small increase in the rest-to-Valsalva hiatal area difference, an increase of mean levator hiatus area during Valsalva maneuver of 1 cm2, and also an increase in the number of women with enlarged levator hiatus at 12 months postpartum compared with pregnancy. Furthermore, when comparing the two delivery groups, no significant differences were found in bladder neck mobility, enlarged levator hiatus, and the rest-to-Valsalva hiatal area difference 1 year postpartum. This could indicate a trend toward persistent pregnancy-induced changes, as found in other studies.14,24 However, these findings did not reach statistical significance in our population using linear mixed models.

The clinical implications of our findings suggest that most women can be reassured that recovery takes place after delivery, although they must be aware that some changes may persist.

We found that the levator ani muscle has a large potential for recovery from both pregnancy and delivery in the majority of women. We found no evidence that changes in levator ani muscle morphology seen during pregnancy persisted 1 year postpartum. However, the fact that the levator ani muscle has undergone these changes during pregnancy might make the pelvic floor less able to withstand the development of pelvic floor disorders when other risk factors are added later in life.